A conventional two-staged projecting bump and its formation method will be described with reference to FIGS. 12A-20.
FIGS. 12A-12F represent a conventional example of a method for forming a two-staged projecting bump.
In the conventional example, as shown in FIG. 12A, a metallic wire 1 of gold, copper, aluminum or solder is passed through a ceramic or ruby capillary 3. A discharging action is brought about between a leading end of the passed metallic wire 1 and an electrode 6, namely, torch, whereby a metallic ball 2 is formed at the leading end of the metallic wire 1.
In FIG. 12B, the metallic ball 2 is pressed onto a preheated electrode 4 of a semiconductor element 8, to which an ultrasonic vibration is impressed. The metallic ball 2 is bonded to the electrode 4 because of the temperature, pressure and the ultrasonic vibration. A reference numeral 5 is a passivation film for protecting an active face of the semiconductor element 8.
In FIG. 12C, the capillary 3 is raised in a vertical direction. Then, the capillary 3 is shifted sideways and descended as in FIG. 12D to bring the metallic wire 1 into contact with the metallic ball 2. The metallic wire 1 is bonded to the metallic ball 2 by temperature and pressure or, temperature, pressure and ultrasonic vibration. The capillary 3 is moved upward as shown in FIG. 12E, and the metallic wire 1 is pulled and broken as in FIG. 12F, whereby a two-staged projecting bump 7 is formed.
According to the above-described prior art, when the metallic ball 2 is formed by the metallic wire 1 through the discharging action, crystal grains of the metallic wire 1 at a part B immediately above the metallic ball 2 (which is denoted as a recrystallized area) are influenced by the heat and become coarse, as shown in FIG. 13. A breaking load of the part B is consequently reduced to nearly half in comparison with a part A not influenced by the heat.
In a case where the two-staged projecting bump 7 is formed through the processes of FIGS. 12A-12F, if a length of the part B which is decreased in breaking load to nearly half due to the influences of the heat as described above is elongated as shown in FIG. 14 with respect to a length of a part C where the metallic wire 1 and metallic ball 2 come in touch with each other, that is, a length C from immediately above the metallic ball 2 to a point D where the metallic wire 1 is to be pulled and broken, a breaking load of the point D where the metallic wire 1 is to be pulled and broken becomes approximately equal to the breaking load of the part B. Therefore, the exact part to be broken cannot be controlled, resulting in the generation of a defective two-staged projecting bump 7a including an excess portion of the metallic wire 1 as illustrated in FIG. 16, in contrast to the two-staged projecting bump 7 of FIG. 15 in a normal shape. The projections of bumps in two stages are thus irregularly shaped as shown in FIG. 17A.
When a conductive paste film 9 is transferred to the defective two-staged projecting bump 7a in a transfer method in the next step as shown in FIG. 17B, the amount of a conductive paste 10 transferred is too much as indicated in FIG. 17C.
In the event that the thus-constituted semiconductor element including the defective two-staged projecting bump 7a is bonded to electrodes 13 of a circuit board 12 in the above state, referring to FIG. 18, there is a likelihood that the excessive amount of conductive paste 10 will cause shortcircuits at portions 11 among the electrodes 13.
The above defective two-staged projecting bump 7a which may cause the shortcircuit accident is also formed by the following reason.
In the process of bonding the metallic wire 1 with the metallic ball 2, depending on a diameter of the used metallic wire 1 or a size of the-metallic ball 2 formed, etc., the metallic wire 1 is caught by the capillary 3 and pressed at E as shown in FIG. 19, which leads to a narrow part (neck) 14 as shown in FIG. 20. The metallic wire 1 is eventually broken starting from the narrow part 14 when pulled and broken. In consequence, there is a likelihood that the defective two-staged projecting bump 7a with the excessive metallic wire 1 is generated as shown in FIG. 16, and too much conductive paste is transferred in the next process, thereby shortcircuiting the electrodes when the semiconductor element is mounted to the circuit board, as mentioned above.
Accordingly, the object of the present invention is to provide a good two-staged projecting bump of a semiconductor element without any variation in shape and a method for forming the bump.